Created attachment 1318[details]
Screenshot of valdiation result.
Steps to repeat:
1. Locate a page of the following characteristics:
a. has XHTML5 markup.
b. has both lang="foo" and xml:lang="foo" on root element
c. has Content-Type set to application/xhtml+xml
d. is served via HTTP or via data URI
2. Check the "Be lax about content-type" option.
2. Validated the page: http://tinyurl.com/adme4qz
(NOTE: The page is parsed as XML.)
Expected result: The lang attribute should be seen as valid.
Actual result: Validator complains that lang is invalid.

(1) A validation counterexample: http://tinyurl.com/bfwfjwf
In the counter example (1), then
* @lang has been deleted (actually: replaced with @data-lang)
* <noscript> (which is invalid in XHTML5) has been added.
# Result: It validates.
# The fact that <noscript> is accepted means (despite the weird
non-acceptance of the @lang attribute) that it is the
HTML-conformance rules that are active, despite the XML parser.
(2) A counter-counterexample: http://tinyurl.com/akwuz2r
In the counter-counterexample (2), then
* Same code as in (1) is used, EXCEPT ...
* … EXCEPT that there is an non-well-formed <hr>
# Result: It doesn't validate due to the non-closed <hr>
# The fact that it doesn't validate despite using a HTML5 preset
is because it nevertheless is the XML parser that is used.